Use Thrash on CD, but not before Faire fire, mangle, lacerate etc. Must always keep debuff/bleed up.

I'll re-word this a little bit.

Use Thrash only to maintain the bleed, but try not to clip it. That is, try not to use Thrash before the bleed falls off unless you've gained a ton of AP in the meantime through trinkets, pot, Vengeance, etc.

Should I get the soft cap or hard cap on expertise as Guardian for a RPS build?

There's really no legitimate reason to not have 15% Expertise once you're beyond 500 item level.

One last question, I'm new to tanking, what should my opener be? Should I open with mangle for the rage/threat. Or is the debuff (10% damage reduction) is more important, so I should open with Thrash and then do as you said to just keep the dot up and not clip.

I pop a pot, eng gloves/trink if applicable, zerk, ff and then mangle spam when opening, after zerk falls off, thrash and then lacerates. Like it has been said, try not to clip thrash, and keep up your dots.

One last question, I'm new to tanking, what should my opener be? Should I open with mangle for the rage/threat. Or is the debuff (10% damage reduction) is more important, so I should open with Thrash and then do as you said to just keep the dot up and not clip.

Depends on the pull ? If it's a 5man heroic and I'm pulling trash maybe I'm just popping incarnation & nv and hitting thrash non stop. Maybe I'm doing a range pull and starting with faire fire. Maybe I'm charging into my target and laying a mangle up first. Maybe I'm pulling a raid boss and I want to zerk at start to cleave multiple targets with mangle.

One last question, I'm new to tanking, what should my opener be? Should I open with mangle for the rage/threat. Or is the debuff (10% damage reduction) is more important, so I should open with Thrash and then do as you said to just keep the dot up and not clip.

The answer will likely change depending upon your specific situation.

I personally try to get all my bleeds rolling before I pop out a Berserk/Incarnation, since I'd rather spend more time using Mangle during those CDs versus initiating/maintaining bleeds and/or debuffs. My basic opener is pulling with FF (mid-charge potentially) -> Mangle -> Thrash -> getting bleed stacks up, since this gets our primary debuffs out. Now, if I'm more concerned about establishing initial threat versus survivability, I'll still pull with FF, but I will likely pop Berserk to Mangle-spam (and if a raid member isn't putting up a bleed, I'll try to sneak Thrash in) and pump out Mauls until I'm satisfied with my threat lead. The answer that works for you will likely be the balance between your threat and your survivability, however threat will likely be the bigger issues on a pull in most cases.

With respect to Thrash and its use for maintaining the bleed, there may be a slight caveat depending on your situation. If you are at 7.5% hit and 15% expertise, by all means, only use Thrash to refresh the bleed as necessary. If you cannot guarantee Thrash will always connect and no one else is putting up Weakened Blows on the boss, using Thrash on cooldown is not a bad thing for survivability... the chances are likely low (but still very possible) that Thrash will connect before the Weakened Blows debuff expires, since it lasts longer than the bleed, but sometimes it's better to be safe than sorry. Unless there is huge variance in Vengeance, the DPS loss from using Thrash on CD won't be drastic, either.